1. Technical Field
This invention relates generally to microwave and millimeter-wave (mm-wave) radio frequency (RF) circuits, and more particularly to transmission line bends for such circuits and the losses they introduce.
2. Description of Related Art
Some mm-wave and microwave RF circuits are integrated on a single dielectric substrate with transmission lines that feed RF between the circuits. Such a transmission line may take the form of a microstrip transmission line, for example, that includes an electrically conductive pattern (a ground plane) on one side of the substrate and a parallel electrically conductive second pattern (a microstrip) on the opposite side of the substrate. RF energy coupled to the transmission line results in an electromagnetic (EM) field between the conductive strip and the ground plane that propagates RF energy along the transmission line within the substrate.
Such transmission lines often include bends that turn the direction of energy propagation (i.e., change the direction of field orientation) from one direction to another. A right angle bend, for example, turns the direction of energy propagation 90-degrees. The problem is that such transmission line bends introduce losses.
One type of loss, called the return loss, relates to the energy being reflected back from the bend. It is represented by the scattering parameter S11 and it is affected by various attributes of the transmission line bend. Capacitance arises through charge accumulation at the corners of a bend, particularly, around the outer point of the bend where electric fields concentrate. Inductance arises also because of current flow constriction. In addition, the change of field orientation at a right angle bend is influenced by mode conversions. These influences significantly increase the return loss.
Focusing on the return loss, several techniques have been investigated in the past for the compensation of microstrip bends in order to reduce the effect of the capacitance and inductance. Doing so improves the voltage standing wave ratio (VSWR) and reduces the return loss. Bends have been mitered and rounded to reduce return loss. In addition, the miter technique removes metal where there is no current flow, and that reduces capacitance and thereby the return loss.
Although the foregoing techniques are helpful in reducing return loss introduced by the transmission line bends, they do not reduce a second type of loss in the insertion loss, namely the losses due to free space radiation and losses due to substrate leakage. The right angle bend is recognized as one of the largest contributors of radiation loss, and detailed analysis of bends and analytical expressions for calculating power loss in right angle bends are available in the literature. However, existing techniques fail to reduce radiation loss adequately for many applications, and so a need exists for a method and bend structure for reducing transmission line bend loss that reduces radiation loss also.
In line with the foregoing, it is an object of this invention to provide a method and bend structure for reducing transmission line bend loss. This object is achieved by providing a bend structure having an electrically conductive strip that forms a bend with at least one inner edge. The inner edge is segmented into multiple non-aligned segments so that its length is increased. Doing so increases the length of the current paths along the inner edge and that helps reduce radiation loss. Stated another way, the invention reduces insertion loss by reducing the phase difference built up in the current and by balancing the fringing field. That also reduces the ground current spreading, thereby reducing the overall radiation.
To paraphrase some of the more precise language appearing in the claims, a transmission line bend structure constructed according to the invention includes a substrate and an electrically conductive pattern on the substrate that forms a transmission line. The electrically conductive pattern includes at least one strip that forms a bend from a first direction to a second direction different from the first direction. The bend includes at least one inner edge and an oppositely disposed outer edge. The inner edge has been modified to include a plurality of segments (curved or straight) so that the inner edge is physically at least as long as the outer edge in order to better match electrical length and thereby reduce transmission line loss.
The technique works as well for T-type junctions. The T-type junction includes oppositely disposed first and second inner edges, the first inner edge extending between first and second end points of the first inner edge, and the second inner edge extending between first and second end points of the second inner edge. The first inner edge includes a first plurality of non-aligned segments that result in the first inner edge having a length greater than a straight line segment between the first and second end points of the first inner edge, and the second inner edge includes a second plurality of non-aligned segments that result in the second inner edge having a length greater than a straight line segment between the first and second end points of the second inner edge. That increases current path lengths along the first and second inner edges in order to help reduce transmission line loss.
In line with the foregoing, a method of designing a transmission line bend structure according to the invention includes the step of providing a preliminary bend structure design having an electrically conductive pattern that includes a strip with at least one inner edge extending along a circuitous path between first and second inner edge end points on the strip. The method proceeds by producing a computer simulation or measurements of the preliminary bend structure design that provides simulation information indicative of transmission line loss characteristics of the preliminary bend structure design. Then, the designer adjusts the circuitous path according to the simulation information to produce an improved bend structure design with reduced transmission line loss.
Thus, the invention may be said to adjust current phase in order to avoid current dipoles in the strip that would otherwise contribute to radiation loss. The energy savings realized is very important at high frequencies and occurs with no detrimental effects in performance. In addition, with radiation loss substantially reduced, antenna patterns from multi-patch antennae can be improved. This allows better prediction of losses and radiation patterns for multi-element printed arrays. The following illustrative drawings and detailed description make the foregoing and other objects, features, and advantages of the invention more apparent.